The invention relates to a security paper with security features in the form of luminescing substances and a process for authenticity determination.
The term "security paper" it is to be understood includes bank notes, check forms, shares and stamps as well as passes, credit cards, check cards, passports, air tickets and other certificates and documents.
The rendering of security papers secure against forgery by luminescing substances has already been known for a along time. Already in German Pat. No. 449 133 from the year 1925 and German Pat. No. 497 037 from the year 1926 the introduction of luminescing substances into security papers is described, wherein the luminophores are excitable with ultraviolet or other invisible radiation and emit in the visible region.
In U.S. Pat. No. 3,473,027 coding inks are described with photoluminescent components on the basis of host lattices doped with rare earth metal ions which can be codoped with one or more dopants according to U.S. Pat. No. 3,525,698. The excitation takes place in the UV-region and short wave visible region and the emission in the visible or IR-region, wherein the emissions in the IR-region can be referred to for broadening the usable spectral range.
The rare earth luminophores described in German Offenlegungsschrift No. 25 47 768 are excited in the IR-region and emit in the visible region.
The use of luminophores for rendering security papers secure is further described in German Offenlegungsschrift No. 15 99 011.
In known security paper luminescing substances, luminophores are chosen the emission of which takes place at a comparatively great spectral distance from the excitation, in order to ensure that the emission characteristics can be determined without the disturbing influence of the excitation light.
Further, in patent literature, numerous proposals for the modification of luminophores, have been made. Examples of such proposals include the combination of a luminophore with other substances, coating a luminophore and encapsulation of a luminophore. These proposals relate to varied purposes including the changing of the spectral regions.
Thus, it has been suggested to improve the chemical resistance of luminophores by covering them with certain substances. In the case of luminescence screens for the multi-color illustration of images, part of the luminescent material has been coated with a barrier layer. In the manufacture of cathode ray tubes for color television receivers it is known to coat the luminescent materials with pigments in order to increase the contrast.
For improving the picture on screen tubes for color television receivers it is further known to suppress undesired emissions of the luminescent materials by pigment coating. In this connection attention can be directed, for example, to German Offenlegungsschrift No. 27 54 369 and U.S. Pat. No. 4,152,483.
It is also known, for example, from British Pat. No. 1 484 471 to broaden the excitation region of a luminophore by combination with a second luminophore.
Further, in German Offenlegungsschrift No. 21 01 120 the coating of luminophores with multiple dielectric layers is described in order to suppress a part of the emission spectrum and in this way to increase the intensity at other wavelengths.
In German Offenlegungsschrift No. 15 99 011 it has already been proposed to cover luminophores which are to be used for rendering identity cards and the like secure with a foil in order to avoid the security feature being able to be detected with the naked eye.
Finally, it is known from British Pat. No. 1 186 253 to mask indicia with luminescing substances partly with dyestuffs which are opaque to the emission of radiation in order thereby to be able to pick out particular indicia such as letters.
In rendering security papers with authenticity features in the form of luminescing substances secure the numerous modifications of luminophores proposed for various purposes have not found any use. As the effort in connection with authenticity features for security papers lay rather in the choice of suitable luminophores the excitation and emission of which had large spectral shift in order to enable identification in the most simple and sure way possible without disturbance from the excitation light.
An important disadvantage of the luminophores used in security papers accordingly lies in the fact that the luminescence can be determined with commercially available devices and from the luminescence characteristics one can exclude the presence of certain luminophores. It would however be more advantageous if with normal means the luminescence overall could not be able to be determined.